Shear Pin Size

Hi Group,
I've upgraded my Lathe Motor and want to include a shear pin on the
spindle pulley. The reason for this is that a threading wizard
recently decided to rapid traverse the tool into the chuck.
The lathe needed a rebuild after this. I had a 3 phase 4Kw motor
lying around so I decided to replace the 750w spindle motor with it as
the lathe was always underpowered.
I need to work out what diameter shear pin to use on the pulley and
what material. The shear pin will be at a radius of 75mm ( 3" ) from
the centre of the spindle and I want to be able to machine steel (
chip breaking ) at a similar radius with carbide tools.
I would prefer to use aluminium or brass for the pin as the pulley is
cast iron.
The shear pin would be in double shear ( 1 driven plate with a
driving plate on each side ) parallel to the axis of the spindle.
Obviously the loads mentioned are ambiguous and I'm only after a ball
park answer.
Thanks,
Glen.